ACTG 375 - Immunologic and Virologic consequences of Long-term Highly Active Antiretroviral Therapy (HAART) in subjects with Moderately Advanced HIV-1 Disease: A Follow-up Study to ACTG 315: This study will explore whether 1) suppression of HIV replication for one year under the influence of highly active antiretroviral therapy (HAART) will result in improvement of immune function; 2) continued suppression of HIV replication for a second year will result in additional improvement of immune function, and 3) successful viral suppression after one year of HAART will be predictive of continued suppression for a second year. This NIH-sponsored, investigator-initiated follow-up study is a 54-week, Phase II pilot trial that will continue to employ an open-label treatment design. Eligible subjects who completed 48 weeks of study treatment in ACTG 315 will be enrolled. All subjects must maintain the current ACTG 315 treatment regimen (zidovudine [ZDV] + lamivudine [3TC] + ritonavir [RTV] while undergoing a six-week period of intensive immunologic evaluations. After completion of these evaluations, the mean of two realtime viral-load measurements will determine initial baseline viral load values. Alternative medications that are either FDA-approved or investigational will be allowed for subjects who experience suboptimal responses to the regimens offered by this study, if approved by the Protocol Chairs in close consultation with the local investigator. Subjects who choose this option and maintain eligibility requirements will continue to participate and be followed on study. All subjects will be required to maintain a HAART regimen that at a minimum contains three drugs, one of which must be a protease inhibitor. Only ZDV, 3TC, stavudine (d4T), didanosine (ddl) delavirdine (DLV), RTV, and saquinavir (SQV) will be supplied by the study.